The extensibility of WordPress lies in the huge number of available extensions, called plugins. A plugin can be a handy helper, it can simplify your work, it can extend the functionality of the system, or it can add something to the page itself. WordPress works well without plugins, but thanks to them you can tailor its functionality to your needs.
Through plugins, you can deal with different types of forms, calendars, social networking elements, media, galleries, etc. You can also significantly improve WordPress SEO capabilities, or turn what was originally a blogging system into a full-fledged e-commerce system with a single plugin.
With tens of thousands of plugins, the possibilities are almost limitless, but you still need to keep an eye out for updates. It can happen that a developer stops updating his plugin, someone discovers a security hole in it and through it can damage your website. That's why you need to keep not only WordPress up to date, but also all plugins if possible.
We covered some of the most used plugins some time ago in an article on our blog, where you can find four essential plugins for WordPress. Back then, we pointed out, for example, Akismet, which serves as a handy catcher of ubiquitous spam, two plugins addressing WordPress SEO, Google Analytics, and Google Fonts. The second article about plugins for WordPress on our blog was already devoted to rather useful but not key plugins: Favikona, calendar, mobile version, etc.
You can search and download plugins for WordPress directly from the system's administrative interface, from where you can also install them. You can also find them on the official wordpress.org website under this link.